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Which physics law states that you can't open a door under water until pressure evens

  1. Jul 1, 2009 #1
    Tried looking for the law with a web search but couldn't quite figure out what the law was. I would like to know the name of the law and any formulas relating to it. I plan to read up on it in the Wiki dictionary, but I need to know what it is called first.
     
  2. jcsd
  3. Jul 1, 2009 #2
    Re: Which physics law states that you can't open a door under water until pressure ev

    I don't think there's a specific 'law' that says you can't open the door, but you can use F = ma (sort of).

    Think of it this way: can you push open the bathroom door if your 5-yr-old cousin is outside trying to hold it shut? Probably yes, because you are a lot stronger than your 5-yr-old cousin. OTOH, could you push the bathroom door open if three Olympic lifters were standing outside, all trying to hold the door shut and keep you in? Probably not, because they would be stronger than you.

    Pressure is just force per unit area, so for a car under water, you can think of a large force vector pushing the door IN while you are trying to push OUT. As water starts to fill the car, a vector grows in the other direction: water pressure inside the car starts to push OUT. As the pressures inside and outside the car get closer to each other, the net force (sum of the forces IN and OUT) gets closer and closer to zero, so you have less and less to push against. Obviously, when the forces are equal, you don't have any resisting force to push against, so the door comes open easily. However, at some point, the net force on the car door will look more like your 5-yr-old cousin is trying to hold it shut than like three Olympic lifters are, and you will be able to open the door (even when the pressure inside and outside aren't exactly equal).
     
  4. Jul 1, 2009 #3
    Re: Which physics law states that you can't open a door under water until pressure ev

    a good place to start would be the bernoulli equation to determine the water pressure, then you need the atmospheric pressure inside and the area of the hatch.
     
  5. Jul 1, 2009 #4

    russ_watters

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    Staff: Mentor

    Re: Which physics law states that you can't open a door under water until pressure ev

    f=P*a
     
  6. Jul 1, 2009 #5
    Re: Which physics law states that you can't open a door under water until pressure ev

    Fluid mechanics
     
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